The present invention relates to equipment for servicing oil and gas wells and, in particular, to an apparatus and method for protecting blowout preventers from exposure to high pressure and abrasive or corrosive fluids during well fracturing and stimulation procedures while providing direct access to production tubing in the well and permitting production tubing or downhole tools to be run in or out of the well.
Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow to make or keep them economically viable. The servicing of oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand.
The components which make up the wellhead such as the valves, tubing hanger, casing hanger, casing head and the blowout preventer equipment are generally selected for the characteristics of the well and not capable of withstanding the fluid pressures required for well fracturing and stimulation procedures. Wellhead components are available that are able to withstand high pressures but it is not economical to equip every well with them.
There are many wellhead isolation tools used in the field that conduct corrosive and abrasive high pressure fluids and gases through the wellhead components to prevent damage thereto.
The wellhead isolation tools in the prior art generally insert a mandrel through the various valves and spools of the wellhead to isolate those components from the elevated pressures and the corrosive and abrasive fluids used in the well treatment to stimulate production. A top end of the mandrel is connected to one or more high pressure valves, through which the stimulation fluids are pumped. In some applications, a pack-off assembly is provided at a bottom end of the mandrel for achieving a fluid seal against an inside of the production tubing or casing so that the wellhead is completely isolated from the stimulation fluids. One such tool is described in Applicant""s U.S. Pat. No. 4,867,243, which issued Sep. 19, 1989 and is entitled WELLHEAD ISOLATION TOOL AND SETTING TOOL AND METHOD OF USING SAME. The length of the mandrel need not be precise because the location of the pack-off assembly in the production tubing or casing is immaterial, so long as the pack-off assembly is sealed against the inner wall of the production tubing or casing. Consequently, variations in the length of the wellhead of different oil or gas wells are of no consequence.
In an improved wellhead isolation tool configuration, the mandrel in an operative position, requires fixed-point pack-off in the well, as described in Applicant""s U.S. Pat. No. 5,819,851, which issued Oct. 13, 1998 and is entitled BLOWOUT PREVENTER PROTECTOR FOR USE DURING HIGH-PRESSURE OIL/GAS WELL STIMULATION. A further improvement of that tool is described in Applicant""s co-pending U.S. patent application Ser. No. 09/299,551 which was filed on Apr. 26, 1999, now U.S. Pat. No. 6,247,537, and is entitled HIGH PRESSURE FLUID SEAL FOR SEALING AGAINST A BIT GUIDE IN A WELLHEAD AND METHOD OF USING SAME. The mandrel described in this patent and patent application includes an annular sealing body attached to the bottom end of the mandrel for sealing against a bit guide which is mounted on the top of a casing in the wellhead.
This type of isolation tool advantageously provides full access to a well casing and permits use of downhole tools during a well stimulation treatment. A mechanical lockdown mechanism for securing a mandrel requiring fixed-point pack-off in the well is described in Applicant""s U.S. patent application Ser. No. 09/338,752 which was filed on Jun. 23, 1999 and is entitled BLOWOUT PREVENTER PROTECTOR AND SETTING TOOL. The mechanical lockdown mechanism has an axial adjusting length adequate to compensate for variations in a distance between a top of the blowout preventer and the top of the casing of the different wellheads to permit the mandrel to be secured in the operative position even if a length of a mandrel is not precisely matched with a particular wellhead. The mechanical lockdown mechanism secures the mandrel against the bit guide to maintain a fluid seal but does not restrain the mandrel from downwards movement. The force exerted on the annular sealing body between the bottom end of the mandrel and the bit guide results from a combination of the weight of the isolation tool and attached valves and fittings, a force applied by the lockdown mechanism and an upward force exerted by fluid pressures acting on the mandrel.
The wellhead isolation tools described in the above patents and patent applications work well and are in significant demand. However, it is also desirable from a cost and safety standpoint, to be able to leave the tubing string, or as it is sometimes called the xe2x80x9ckill stringxe2x80x9d, in the well during a well stimulation treatment. The above-described wellhead isolation tool is not adapted to support a tubing string left in the well because the weight of a long tubing string may damage the seal between the bottom of the mandrel and the bit guide.
Some prior art wellhead isolation tools are adapted for well stimulation treatment with a tubing string left in the well. For example, Canadian Patent No. 1,281,280 which is entitled ANNULAR AND CONCENTRIC FLOW WELLHEAD ISOLATION TOOL AND METHOD OF USE THEREOF, which issued to McLeod on Mar. 12, 1991, describes an apparatus for isolating the wellhead equipment from the high pressure fluids pumped down to the production formation during the procedures of fracturing and acidizing oil and gas wells. The apparatus utilizes a central mandrel inside an outer mandrel and an expandable sealing nipple to seal the outer mandrel against the casing. The bottom end of the central mandrel is connected to a top of the tubing string and a sealing nipple is provided with passageways to permit fluids to be pumped down the tubing and/or the annulus between the tubing and the casing in an oil or gas well. One disadvantage of this apparatus is that the fluid flow rate is restricted by the diameter of the outer mandrel which must be smaller than the diameter of the casing of the well and further restricted by the passageways in the sealing nipple between the central and outer mandrels. The sealing nipple also blocks the annulus, preventing tools from being run down the wellbore. The passageways in the sealing nipple are also susceptible to damage by the abrasive particle-laden fluids and are easily washed-out during a well stimulation treatment. A further disadvantage of the isolation tool is that the tool has to be removed and re-installed every time the tubing string is to be moved up or down in the well.
Therefore, there exists a need for an improved isolation tool which is adapted for use with a tubing string to be left in the well, or run into or out of the well during a well stimulation treatment.
It is an object of the invention to provide a BOP protector which is adapted to support a tubing string in a wellbore so that the tubing string is accessible during a well treatment to stimulate production.
It is a further object of the invention to provide a BOP protector that permits a tubing string to be moved up and down in the wellbore without removing the BOP protector from the wellhead.
It is another object of the present invention to provide a BOP protector that permits a tubing string to be run into or out of the wellbore without removing the BOP protector from the wellhead.
In accordance with one aspect of the invention, there is provided an apparatus for protecting a blowout preventer from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment to stimulate production. The apparatus is adapted to support a tubing string in a wellbore so that the tubing string is accessible during the well treatment. The apparatus includes a mandrel adapted to be inserted down through the blowout preventer to an operative position. The mandrel has a mandrel top end and a mandrel bottom end. The mandrel bottom end includes an annular sealing body for sealing engagement with a bit guide at a top of a casing of the well when the mandrel is in the operative position. A base member is adapted for connection to the wellhead and includes fluid seals through which the mandrel is reciprocally moveable. The apparatus further comprises a fracturing head, a tubing adapter and a lock mechanism. The fracturing head includes a central passage in fluid communication with the mandrel and at least one radial passage in fluid communication with the central passage. The tubing adapter is mounted to a top end of the fracturing head and supports the tubing string while permitting fluid communication with the tubing string. The lock mechanism for locking the apparatus in a fixed position to inhibit upward movement of the mandrel induced by fluid pressures in the wellbore and downward movement of the mandrel induced by a weight of the tubing string supported by the tubing adapter.
The apparatus preferably includes a mandrel head affixed to the mandrel top end and the fracturing head is mounted to the mandrel head. The lock mechanism preferably includes a mechanical lockdown mechanism which is adapted to inhibit upward movement of the mandrel head induced by fluid pressures when the mandrel is in the operative position and a load transferring mechanism for transferring a substantial part of the weight of the tubing string from the mandrel head to the wellhead to protect the sealing body from the entire weight of the tubing string when the tubing string is supported by the tubing adapter.
More especially, according to an embodiment of the invention, the base member has a central passage to permit the insertion and removal of the mandrel. The passage is surrounded by an integral sleeve having an elongated spiral thread for engaging a lockdown nut that is adapted to secure the mandrel in the operative position. A passage from the mandrel head top end to the mandrel head bottom end is provided for fluid communication with the mandrel and permits the tubing string to extend therethrough. The mandrel head includes a spiral thread for operatively engaging a load transfer nut that is adapted to be rotated down so that a head of the load transfer nut rests against a top of the lockdown nut to transfer the weight of the tubing string from the mandrel head to the base member.
The tubing adapter is configured to meet the requirements of a job. It may be a flange for mounting a BOP to the top of the apparatus so that tubing can be run into or out of the well. Alternatively, the tubing adapter may include a threaded connector to permit the connection of a tubing string that is already in the well.
A blast joint may be connected to the tubing adapter if coil tubing is run into the well. The blast joint protects the coil tubing from erosion when abrasive fluids are pumped through the fracturing head.
In accordance with another aspect of the invention, a method is described for providing access to a tubing string while protecting a blowout preventer on a wellhead from exposure to fluid pressure as well as to abrasive and corrosive fluids during a well treatment to stimulate production. The method comprises:
a) suspending the apparatus above the wellhead;
b) aligning the apparatus with a tubing string supported on the wellhead and lowering the apparatus until a top end of the tubing string extends through the axial passage above the fracturing head;
c) connecting the top end of the tubing string to a top end of the fracturing head, lowering the tubing string and the apparatus until the apparatus rests on the wellhead, and mounting the base member to the wellhead;
d) opening the blowout preventer;
e) lowering the tubing string and the fracturing head to stroke the mandrel bottom end down through the blowout preventer, and adjusting a lock mechanism until the mandrel is in an operative position in which the annular sealing body is in fluid sealing engagement with a bit guide mounted to a top of the casing of the well;
f) adjusting the lock mechanism to lock the mandrel in the operative position and to transfer weight of the tubing string and the apparatus to the wellhead so that the sealing body is not compressed against the bit guide by a full weight of the tubing string.
In accordance with a further aspect of the invention, a method is described for running a tubing string into or out of a well while protecting a first blowout preventer on a wellhead of the well from exposure to fluid pressure as well as to abrasive and corrosive fluids during a well treatment to stimulate production. The method related to the use of the above-described apparatus comprises:
a) mounting the base member of the apparatus to the wellhead;
b) closing at least one second blowout preventer which is mounted to an adapter flange a top the fracturing head;
c) opening the first blowout preventer;
d) lowering the fracturing head to stroke the mandrel bottom end down through the blowout preventer, and adjusting a lock mechanism until the mandrel is in an operative position in which the annular sealing body is in fluid sealing engagement with a bit guide mounted to a top of the casing of the well;
e) adjusting the lock mechanism to lock the mandrel in the operative position and to transfer weight of the tubing string and the apparatus to the wellhead so that the sealing body will not be compressed against the bit guide by a full weight of the tubing string; and
f) running the tubing string into or out of the well through the at least one second blowout preventer.
A primary advantage of the invention is the capability to support a tubing string in a wellbore during the well stimulation treatment. This provides direct access to both the tubing string and the well casing so that the use of the apparatus is extended to a wide range of well service applications.
A further advantage of the invention is to permit a maximum flow rate into the well during a stimulation treatment because the mandrel has a diameter at least as large as that of the casing of the well. Furthermore, the apparatus permits the tubing string to be moved up and down, or run in or out of the well without removing the apparatus from the wellhead. The tubing string can even be moved up or down in the well while well treatment fluids are being pumped into the well. Labour and the associated costs are thus reduced.